<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title>文档标题</title>
    <style>
        .parent-wraper{

            border: 1px solid blue;
            height: 100px;
        }
       .child-wraper{
            border: 1px solid red;
            height: 50px;
            margin: 20px 0px 20px 0px;
       }
    </style>
</head>
<body>
<div class="parent-wraper">
    <div class="child-wraper"></div>
</div>
</body>
</html>